1. Field of the Invention
The present invention relates in general to a reactor and a process for the preparation of hydrocarbons from synthesis gas, typically known as the Fischer-Tropsch process. More particularly, this invention pertains to a three phase slurry bubble column reactor that can maximize the production rate per unit volume of the reactor.
2. Prior Art
The Fischer-Tropsch process is the second step of a two step process that converts natural gas to liquid fuels. The first step in the conversion process transforms carbonaceous feedstock such as natural gas, coal, petroleum coke, heavy oils, biomass, landfill gas, biogas and municipal waste into a synthesis gas comprised of carbon monoxide and hydrogen. The second step, which is commonly known as the Fischer-Tropsch process, converts the synthesis gas over a suitable catalyst into a wide range of hydrocarbons such as methanol, mixed alcohols, olefins, paraffinic hydrocarbons and mixtures thereof. These materials are useful for production of chemical and fuel products.
The Fischer-Tropsch process is a reaction that aims to produce hydrocarbons that contain five or more carbon atoms per molecule. Therefore, the catalyst and operating conditions are selected to reduce the formation of methane.
Various types of reactors have been employed for Fischer-Tropsch and related synthesis reactions. Fischer-Tropsch reactors include fixed bed reactors, fluidized bed reactors, and gas-agitated three phase reactors often called “slurry bubble column” reactors. Slurry bubble column reactors operate by suspending catalytic particles in liquid hydrocarbons creating a “slurry” mixture, and then pumping synthesis gas reactants through the bottom of the reactor to create small gas bubbles. As the gas bubbles containing reactants rise in the reactor, they absorb into the slurry mixture and diffuse into the catalyst particles where they are converted into gaseous and liquid hydrocarbon products. The gaseous products are collected at the top of the reactor and the liquid products are separated from the slurry by a variety of separation techniques.
Slurry bubble column reactors are favored over fixed bed reactors because they utilize smaller catalyst particles and have better heat and mass transfer capabilities. However, the slurry bubble column reactor is both costly and difficult to scale up.
One of the main constraints with current technology for three phase reactors, and in particular Fischer-Tropsch slurry bed reactors, is the relationship between the reactor vessel diameter and the reactor vessel height. Slurry bubble column reactors can operate at different velocities. When gas is fed into the reactor at a low linear velocity, carbon monoxide conversion proceeds at a relatively high rate as compared with the reactor height, thereby reaching high CO conversions at relatively low reactor heights. As a result, only a small portion of the CO will remain to react in the upper portion of the reactor. Because a water molecule is formed as every CO molecule is converted, the high rate of CO conversion also increases the partial pressure of water. High water partial pressures increase the rate of deactivation of Fischer-Tropsch catalysts and are therefore undesirable. To counter these undesirable effects, a Fischer-Tropsch slurry bubble column reactor would either need to have an impractical, low height or operate at conditions providing low Fischer-Tropsch conversion rates, such as low temperature, to avoid reaching higher partial pressure of water. The reactor would thus have a much lower total productivity than desired.
Another approach to avoid the high water partial pressure and/or have a higher total productivity is to operate at high gas linear velocities, which would alleviate the high percentage of CO conversion at relatively low reactor heights. While this reactor will have a higher total productivity than the other approach described earlier, the percentage of CO conversion per reactor unit volume will be lower than can be achieved by running the reactor at lower gas linear velocities. Also, as the gas linear velocity increases, the volume percentage of gas in the reactive section of the reactor (gas hold up or GHU) increases. Due to this increase in gas hold up, there is an upper limit for the gas linear velocity, above which, the reactor productivity actually declines. Reactors that operate at higher gas linear velocities will typically have diameters between 10 and 11 meters, with a height of 30 meters or more. Typical operating conditions for slurry bubble column Fischer-Tropsch reactors are at temperatures between 180 and 260° C., and pressures between 10 and 40 bar, with the reactor vessel being a pressure vessel. Because of the operating conditions, and the large reactor shell dimensions, there are very few places in the world where these vessels can be constructed. The current delivery time for reactor shell production is years rather than months. This has a severe negative effect toward the application of the Fischer-Tropsch slurry bed technology worldwide.
Another problem with current slurry bed reactor technology is the complex hydrodynamic regime inside the slurry bed units. In order to achieve high productivity, reactors need to have large diameters and heights. Under these conditions it is difficult, costly and complex to predict the reactor hydrodynamic profile inside the large commercial unit. Very few companies worldwide can afford to construct demonstration plants in order to assess the complex hydrodynamic regime inside slurry bed units.
Yet, another problem with current slurry bed reactor technology is the difficulty with separating the catalyst from the liquid hydrocarbon products. The current approach is to develop attrition-resistant catalysts. If the catalysts attrite, small particles are created which lowers the efficiency of the solid-liquid separation system, irrespective of whether the assembly consists of filters, settlers, hydro cyclones, magnetic techniques, or a combination of the above techniques. Catalysts also have to operate in a large diameter reactor of over 20 meters high, at linear velocities that result in a churn turbulent flow hydrodynamic regime. Under these conditions, the catalyst particles can travel at linear velocities close to 2 m/s and at higher velocities in the central section of the reactor. The central section of the reactor has a higher than average gas hold up and bubble size and therefore high turbulence. Therefore the particle to particle collisions and collisions with reactor internals such as cooling coils caused by catalysts moving at high linear velocities in the central section of the reactor will inevitably result in catalyst attrition. Under these conditions, an “attrition resistant” catalyst is necessary.
Accordingly, it is a principal object and purpose of the present invention to provide a three phase reactor, applicable in the Fischer-Tropsch process, which solves or minimizes the problems described above. The present invention provides a slurry bubble column reactor that has high productivity per unit volume which is largely independent from the total productivity per reactor.
The reactor of the present invention operates in a regime which lowers the rate of catalyst attrition for any given catalyst and is significantly easier to scale-up for commercial application.